Fix fields_for with :index missing index on form option helpers

In FormHelper helpers [text_field, etc] the :index option from fields_for is merged into options for to_text_field_tag [etc] tag. The FormOptionHelpers do not do this and the select tag helpers end up missing the index set in the fields_for. This patch just replicates the @default_options merge from FormHelper#objectify_options as FormHelper#defaultify_options which merges @default_options with the html_options.